DESCRIPTION

Event timers are responsible for generating interrupts at specified time or periodically, to
run different time-based events. Subsystem consists of three main parts:
Drivers Manage hardware to generate requested time events.
Consumers sys/kern/kern_clocksource.c uses event timers to supply kernel with hardclock(),
statclock() and profclock() time events.
Glue code sys/sys/timeet.h, sys/kern/kern_et.c provide APIs for event timer drivers and
consumers.

DRIVER API

Driver API is built around eventtimer structure. To register its functionality driver allo‐
cates that structure and calls et_register(). Driver should fill following fields there:
et_name Unique name of the event timer for management purposes.
et_flags Set of flags, describing timer capabilities:
ET_FLAGS_PERIODIC Periodic mode supported.
ET_FLAGS_ONESHOT One-shot mode supported.
ET_FLAGS_PERCPU Timer is per-CPU.
ET_FLAGS_C3STOP Timer may stop in CPU sleep state.
ET_FLAGS_POW2DIV Timer supports only 2^n divisors.
et_quality Abstract value to certify whether this timecounter is better than the others.
Higher value means better.
et_frequency Timer oscillator's base frequency, if applicable and known. Used by consumers
to predict set of possible frequencies that could be obtained by dividing it.
Should be zero if not applicable or unknown.
et_min_period, et_max_period
Minimal and maximal reliably programmable time periods.
et_start Driver's timer start function pointer.
et_stop Driver's timer stop function pointer.
et_priv Driver's private data storage.
After the event timer functionality is registered, it is controlled via et_start and et_stop
methods. et_start method is called to start the specified event timer. The last two argu‐
ments are used to specify time when events should be generated. first argument specifies
time period before the first event generated. In periodic mode NULL value specifies that
first period is equal to the period argument value. period argument specifies the time
period between following events for the periodic mode. The NULL value there specifies the
one-shot mode. At least one of these two arguments should be not NULL. When event time
arrive, driver should call et_event_cb callback function, passing et_arg as the second argu‐
ment. et_stop method is called to stop the specified event timer. For the per-CPU event
timers et_start and et_stop methods control timers associated with the current CPU.
Driver may deregister its functionality by calling et_deregister().
If the frequency of the clock hardware can change while it is running (for example, during
power-saving modes), the driver must call et_change_frequency() on each change. If the
given event timer is the active timer, et_change_frequency() stops the timer on all CPUs,
updates et->frequency, then restarts the timer on all CPUs so that all current events are
rescheduled using the new frequency. If the given timer is not currently active,
et_change_frequency() simply updates et->frequency.

CONSUMER API

et_find() allows consumer to find available event timer, optionally matching specific name
and/or capability flags. Consumer may read returned eventtimer structure, but should not
modify it. When wanted event timer is found, et_init() should be called for it, submitting
event and optionally deregister callbacks functions, and the opaque argument arg. That
argument will be passed as argument to the callbacks. Event callback function will be
called on scheduled time events. It is called from the hardware interrupt context, so no
sleep is permitted there. Deregister callback function may be called to report consumer
that the event timer functionality is no longer available. On this call, consumer should
stop using event timer before the return.
After the timer is found and initialized, it can be controlled via et_start() and et_stop().
The arguments are the same as described in driver API. Per-CPU event timers can be con‐
trolled only from specific CPUs.
et_ban() allows consumer to mark event timer as broken via clearing both one-shot and peri‐
odic capability flags, if it was somehow detected. et_free() is the opposite to et_init().
It releases the event timer for other consumers use.
ET_LOCK() and ET_UNLOCK() macros should be used to manage mutex(9) lock around et_find(),
et_init() and et_free() calls to serialize access to the list of the registered event timers
and the pointers returned by et_find(). et_start() and et_stop() calls should be serialized
in consumer's internal way to avoid concurrent timer hardware access.